Revealing hydrodynamic controls on algal bloom outbreaks in the Three Gorges Reservoir backwater area: Insights from triple oxygen isotopes analysis

The Three Gorges Dam, as the largest hydropower project in the world, has created a relatively static water environment that promotes the frequent occurrence of algal blooms in the backwater area, thereby exerting negative impacts on the ecological environment and socio-economic benefits of the reservoir. The triple oxygen isotope technique builds upon the capabilities of conventional hydrogen and oxygen isotopes for tracking water exchange processes between different water bodies, while enhancing the unique advantage of δ¹⁷O in resisting equilibrium fractionation effects induced by temperature variations. This innovative approach is applied to the Three Gorges Reservoir specifically to explore the interactions between hydrodynamics and algal bloom formation. The results show that the triple oxygen isotope composition of the mainstream is lower than that of its tributaries, with higher isotope values observed in upstream tributaries. End-member mixing analysis indicates that the average contribution of mainstream water to individual tributaries increases progressively as the river approaches the dam. Furthermore, based on the Ward's classification method, tributary sampling sites with isotope values similar to those of the mainstream exhibit significantly higher frequencies of algal blooms than local rain-fed rivers, demonstrating that channels characterized by long water residence times and low flow velocities provide favorable conditions for algal bloom outbreaks. This study highlights the influence of reservoir-induced hydrological alterations on algal dynamics and provides scientific guidance for the effective management of ecological environments in backwater regions.